How can I enable/disable ethernet in Android 6 Marshmallow

I have a M9X Android box with Android 6 and I cannot find an option
how I can disable the Ethernet connection. I only have the option to
turn on or off the Wifi but I need to be able to chose between using
Ethernet or Wifi but right now, since have a cable connected it will
always use Ethernet.

Well, most devices will prefer wired over wireless internet, because it's often faster, more reliable and less power-hungry. Android by default does not include toggles for ethernet because the operating system is mostly aimed at mobile devices, not set top boxes. If the android version shipped by the box maker does not include a way of toggling this, then you might want to look at the rooting scene for the device, and see if someone made a custom rom or app for it.